Due to the characteristics of thin profile and low power consumption, liquid crystal displays (LCDs) are widely used in electronic products, such as portable personal computers, digital cameras, projectors, and the like. Generally, LCD panels are classified into transmissive, reflective, and transflective types. A transmissive LCD panel uses a back-light module as its light source. A reflective LCD panel uses ambient light as its light source. A transflective LCD panel makes use of both the back-light source and ambient light.
As known in the art, a color LCD panel 1 has a two-dimensional array of pixels 10, as shown in FIG. 1. Each of the pixels comprises a plurality of sub-pixels, usually in three primary colors of red (R), green (G) and blue (B). These RGB color components can be achieved by using respective color filters. FIG. 2 illustrates a plan view of the pixel structure in a conventional transflective liquid crystal panel, and FIGS. 3a and 3b are cross sectional views of the pixel structure. As shown in FIG. 2, a pixel can be divided into three sub-pixels 12R, 12G and 12B, and each sub-pixel can be divided into a transmission area (TA) and a reflection area (RA). In the transmission area as shown in FIG. 3a, light from a back-light source enters the pixel area through a lower substrate 30 and goes through a liquid crystal layer, a color filter R and the upper substrate 20. In the reflection area, light from above an upper substrate 20 encountering the reflection area goes through the upper substrate 20, the color filter R and the liquid crystal layer before it is reflected by a reflective layer or electrode 52. Alternatively, a non-color filter (NCF) is formed on the upper substrate 20, corresponding to part of the reflective area, as shown in FIG. 3b. 
As known in the art, there are many more layers in each pixel for controlling the optical behavior of the liquid crystal layer. These layers may include a device layer 50 and one or two electrode layers. For example, a transmissive electrode 54 on the device layer 50, together with a common electrode 22 on the color filter, is used to control the optical behavior of the liquid crystal layer in the transmission area. Likewise, the optical behavior of the liquid crystal layer in the reflection area is controlled by the reflective electrode 52 and the common electrode 22. The common electrode 22 is connected to a common line. The device layer is typically disposed on the lower substrate and comprises gate lines 31, 32, data lines 21-24 (FIG. 2), transistors, and passivation layers (not shown). Furthermore, a storage capacitor is commonly disposed in the device layer 50 to retain the electrical charge in the sub-pixel after a signal pulse in the gate line has passed. An equivalent circuit of a typical sub-pixel (m, n) having a transmission area and a reflection area is shown in FIG. 4. In FIG. 4, CLC1 is the capacitance mainly attributable to the liquid crystal layer between the transmissive electrode 54 and the common electrode 22, and CLC2 is the capacitance mainly attributable to the liquid crystal layer between the reflective electrode 52 and the common electrode 22. C1 is the storage capacitor and COM denotes the common line.
As it is known in the art, an LCD panel also has quarter-wave plates and polarizers.
In a single-gap transflective LCD, one of the major disadvantages is that the transmissivity of the transmission area (transmittance, the V-T curve) and the reflectivity in the reflection area (reflectance, the V-R curve) do not reach their peak values in the same voltage range. As shown in FIG. 5, the V-R curve is peaked at about 2.8V, while the “flat” section of the V-T curve is between 3.7V and 5V. The reflectance experiences an inversion while the transmittance is approaching its higher values.
In prior art, this reflectivity inversion problem has been corrected by using a double-gap design wherein the gap at the reflection area is about half of the gap at the transmission area. While the double-gap design is effective in principle, it is difficult to achieve in practice mainly due to the complexity in the fabrication process. Other attempts, such as manipulating the voltage levels in the transmission and the reflection areas and coating the reflective electrode by a dielectric layer, have been proposed. For example, the voltage level in the reflection area relative to that in the transmission area is reduced by using capacitors. As shown in FIG. 6, a separate capacitor CC is connected in series to CLC2. As such, the voltage level on the reflective electrode in reference to the common line voltage level VCOM1 is given by:
                              V                      CLC            ⁢                                                  ⁢            2                          =                ⁢                  Vcc          -                      Vcom            ⁢                                                  ⁢            1                                                  =                ⁢                              Cc                          (                                                C                                      L                    ⁢                                                                                  ⁢                    C                    ⁢                                                                                  ⁢                    2                                                  +                Cc                            )                                *                      (                                          V                data                            -                              Vcom                ⁢                                                                  ⁢                1                                      )                              where Vdata is the voltage level on the data line.
By adjusting the ratio CC/(CCL2+CC), it is possible to shift the peak of the reflectance curve toward the higher voltage end so as to match the flatter region of the transmittance curve, as shown in FIG. 7a. As such, the inversion in the reflectance relative to the transmittance can be avoided.
However, while the transmittance starts to increase rapidly at about 2.2V, the reflectance remains low until about 2.8V. In this low brightness region, the discrepancy in the transmittance and reflectance also causes the discrepancy between the gamma curve associated with the transmittance and the gamma curve associated with the reflectance, as shown in FIG. 7b. FIG. 7b shows the transmittance and reflectance as a function of gamma level. Such discrepancy in the gamma curves degrades the view quality of a transflective LCD panel.
It is thus advantageous and desirable to provide a method to reduce the discrepancy between the gamma curve associated with the transmittance and the gamma curve associated with the reflectance.